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Effect of prostate-specific membrane antigen (PSMA) radioimmunotherapy on circulating tumor cell (CTC) count.
Background: CTC counts with CellSearch methodology are prognostic; changes following therapy may be associated with improved outcomes. Since FDA clearance, baseline and follow up CTC counts were included in studies of radiolabeled J591. The effect of J591 alone (without an effector molecule) is unknown. Methods: Our phase II single-dose 177Lu-J591 (Cohort 3), phase I fractionated dose 177Lu-J591 (expansion cohorts), and phase I fractionated 177Lu-J591 + docetaxel (all pts) trials were analyzed. Any pt with baseline and at least 1 follow-up CTC count (CellSearch) was included in the analysis. A retrospective subset of patients undergoing J591-based imaging without an effector molecule attached was also analyzed. Results: 48 pts received 177Lu-J591 with prospectively measured CTC counts with median age of 73.7 years. 26 of 48 (54.2%) had unfavorable (≥5 CTCs/7.5 mL blood) baseline CTCs (median 29, range 5-449 CTCs/7.5 mL). Of the 23 with ≥5 baseline CTCs and available 4-6 wk post-treatment counts, 22 (95.6%) had declines of 15-100%, with 12 (52.2%) converting to favorable counts. Of the 18 with ≥5 baseline CTCs and available 12-14 wk post-treatment counts, 10 (55.5%) had declines of 8-100%, with 7 (38.9%) converting to favorable counts. Of the 22 of 48 (45.8%) with baseline CTC counts <5, 1 became unfavorable at 4-6 wks, later re-converting to favorable (early control rate of 95.5%) and 1 became unfavorable at 12-14 wks (sustained control rate of 95.5%). 7 men with unfavorable counts underwent imaging with 20 mg of J591 without an attached effector molecule; 4 (57%) demonstrated decline in CTCs following the imaging dose, with 1 stable and 2 continued to increase prior to subsequent therapy. Conclusions: In addition to the favorable PSA declines previously reported, radiolabeled J591 results in frequent favorable changes in CTC counts, with >90% of men with baseline unfavorable counts receiving 177Lu-J591 demonstrating CTC count decline or control in pts with baseline favorable counts. A small retrospective subset suggests that CTC counts may also be controlled with naked (unlabeled) anti-PSMA monoclonal antibody and a prospective study investigating this phenomenon will begin enrollment. Clinical trial information: NCT00195039, NCT00538668, NCT00916123.